try a smaller size card (8gb, or 16gb), and try a different brand or different speed card, and format the card inside the camera, but if that isn't good, then format the card as FAT32 with 64 kb allocation unit inside a computer http://www.ridgecrop.demon.co.uk/guiformat.htm

I did reformat my SD card ( which was 32kB alloc ) to 64 kB alloc, the problem was worst. This give me the idea to reformat with smaller allocation block.
When formatted with 4kB block, the 720p footage look a lot better, in a very short test movie.